The wedding scene in which Mark gives Juliet and Peter the best wedding present ever - a performance of "All You Need Is Love" with full choir, orchestra, and guitars and drums.
Mark, Juliet, cue cards and fake carol singers, as Mark finally says what he has wanted to say to Juliet since he first met her. THAT is his CMoA.
Sam's wild run through the departures area of Heathrow Airport to catch Joanna before she boards her flight back to New York; after pulling all sorts of gymnastic manoeuvres (especially in the uncut version of the sequence) and benefitting from the gate staff being distracted by Billy Mack's video striptease, he finally catches up with her just before she walks down the jetway to the aircraft... and, better still, learns that she has been just as smitten by him as he has by her, and really was directing her performance of "All I Want for Christmas Is You" to him as a way of telling him this.
On a related note, Olivia Olson's performance of "All I Want For Christmas Is You." Even better when you learn she actually had to tone down her singing skills so people wouldn't assume she was dubbed.
David's driver pulling out his operatic singing voice for Christmas carols.
This movie helped kick start an entire genre of film (Ensemble cast in a rom-com). How awesome is that?
The Prime Minister's Speech. In which he verbally burns the US President, gives a massive shout-out to the great history of Britain, and serves notice that the UK isn't going to simply take what the US dishes out anymore. It was instantly beloved across Britain and became the centerpiece of a "look how awesome the UK is" advertising campaign.